Introduction to LEDs’ Basics

By Scott Patterson

Light bulbs
are seemed everywhere, and there are more than 20 billion light bulbs around the world right now. On an average that for every one person, there would be three light bulbs. In the future, we can expect a replacement of all the current lightbulbs with those of LEDs. The reason for the occurrence of this replacement is due to the fact that LEDs’ efficacy is multiple times higher than those of incandescent light bulbs. LED is possible in reproduce the theoretical limit of the conversion from electricity to light set by physics. LEDinside will dedicate a large resource contributing to the information about the practical aspects of LEDs and their lighting designs.

Before we go further, it is important for us to understand the concept of LED. This LEDinside Knowledge Kit’s main objective is to tell you all practical uses of LEDs. Here we will not trouble you with the technical terms such as GaInP, GaP, etc. We want to quickly and emphatically, tell you what is LED exactly.

LED stands for light emitting diode, just by looking at the term, you can already know a thing or two about LED that it is a diode. A diode is a device conducts current in one direction but not on the other. This summarizes what LED can do. LEDinside will explore more on its electrical behavior in future. What we wants to take away from here is knowing that LED has a much higher forward voltage than normal diodes used in electronics.

While a common diode like 1N4148 has a drop of approximate 0.7V, a LED may has a drop of 3.6V. The reason behind this difference in electrical characteristics is due to the fact LED is not made of silicon. However on other characteristics, LEDs are very similar to those of other diodes.

Also from the words light and emitting you get to know a lot more about LED. All diodes emit some or a little bit of light. Looking at a integrated circuit or IC, and use a scanner to find which part of the circuit is emitting light or conducting current. IC engineers use this method to debug their IC designs. However, the amount of light emitted by ICs is very small. LEDs, on the other hand, are designed and optimized for this very purpose. This is also why LEDs have a much higher forward voltage. Normal diodes have been modified to minimize their forward voltage and optimize their reverse breakdown voltage. LEDs are designed to produce the most amount of the right color of light while consume the least power. Forward voltage here by itself does not contribute to the process. However, forward voltage does explain how much of the power LED dissipates as heat, which we will discuss more about the next time.

Present day thinking can generally divide LEDs into two categories: those of small devices and of power devices. Small has become prevalently adapted starting in the 1970s. They come in various colors such as red, blue, green etc. They are the small T1 devices approximately 5 mm each. It is so prevalent that they are sold tens of billions in units each year. They are used in the manufacturing of cell phone backlights, elevator pushbuttons, light bulbs, road signage, and a lot more. 

What is notable about these small devices is their power level, or their drive current. A typical red small LED has a drive current of 20 mA with a forward voltage of 2.2 V, it only 44 mW in power. (The efficacy is so low that this is generally equal to the heat dissipated.) Small white LEDs have a higher forward voltage of 3.6V corresponding to 72mW and some small LEDs can be run to 100mA. Generally, these types of LEDs are used as indicators and not for lighting purpose. It takes about 14 of these LEDs to make a 1 W flashlight and at least hundreds of them to make a dim florescent replacement. Our focus here is on power devices. Power device are usually from 1 to 3 W and run at 350 mA. Their actual semiconductor part of LED as opposed to its package, the dice is significant larger than those of small LEDs, but it is not necessary for their footprint to be. These devices are commonly used in lighting than as indicators.

Applications are vast such as flashlights, incandescent light bulb replacements, screen backlights, projector lights, headlights, and gradually penetrate through every known lighting use.

White LEDs and color LED are very similar with only one obvious difference being the difference between forward voltages. The reason for this difference in forward voltages is due to the difference in the semiconductor material used to generate various color of lights such as red, yellow and blue.

Furthermore, white light cannot be manufacture using one single material or not yet an engineered material been created to produce white light. If you have experienced with a prism, you would know that white light consists of various different colors. Thus, white light can be recreate using two general methods, one being using phosphor to convert blue light into white light, and the other being the combination of different color LEDs.

The phosphor method is the most common. A typical wavelength of a blue light generated by LEDs is around 435 nm. Using blue light is the optimum choice because blue light can be absorbed by phosphor and reemitted as a broad spectrum of light approximating white light. Through this process light coming out from the phosphor will be lower than the energy when entering phosphor. Those disappeared energy is dissipated in a form of heat. In order to generated all the colors we, human can see in the spectrum, light entering has to be of higher energy that also means having a shorter wavelength, shorter than the color with highest energy here.

For humans, the visible wavelengths is around 400 to 700 nm, and at 435 nm, the blue LED is the most energy efficient choice in generating white light using the phosphor method.

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